/*-------------------------------------------------------------------------

 *

 * stack_depth.c

 *    Functions for monitoring and limiting process stack depth

 *

 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group

 * Portions Copyright (c) 1994, Regents of the University of California

 *

 *

 * IDENTIFICATION

 *    src/backend/utils/misc/stack_depth.c

 *

 *-------------------------------------------------------------------------

 */

#include "postgres.h"

#include <limits.h>

#include <sys/resource.h>

#include "miscadmin.h"

#include "utils/guc_hooks.h"

/* GUC variable for maximum stack depth (measured in kilobytes) */

int     max_stack_depth = 100;

/* max_stack_depth converted to bytes for speed of checking */

static ssize_t max_stack_depth_bytes = 100 * (ssize_t) 1024;

/*

 * Stack base pointer -- initialized by set_stack_base(), which

 * should be called from main().

 */

static char *stack_base_ptr = NULL;

/*

 * set_stack_base: set up reference point for stack depth checking

 *

 * Returns the old reference point, if any.

 */

pg_stack_base_t

set_stack_base(void)

{

#ifndef HAVE__BUILTIN_FRAME_ADDRESS

  char    stack_base;

#endif

  pg_stack_base_t old;

  old = stack_base_ptr;

  /*

   * Set up reference point for stack depth checking.  On recent gcc we use

   * __builtin_frame_address() to avoid a warning about storing a local

   * variable's address in a long-lived variable.

   */

#ifdef HAVE__BUILTIN_FRAME_ADDRESS

  stack_base_ptr = __builtin_frame_address(0);

#else

  stack_base_ptr = &stack_base;

#endif

  return old;

}

/*

 * restore_stack_base: restore reference point for stack depth checking

 *

 * This can be used after set_stack_base() to restore the old value. This

 * is currently only used in PL/Java. When PL/Java calls a backend function

 * from different thread, the thread's stack is at a different location than

 * the main thread's stack, so it sets the base pointer before the call, and

 * restores it afterwards.

 */

void

restore_stack_base(pg_stack_base_t base)

{

  stack_base_ptr = base;

}

/*

 * check_stack_depth/stack_is_too_deep: check for excessively deep recursion

 *

 * This should be called someplace in any recursive routine that might possibly

 * recurse deep enough to overflow the stack.  Most Unixen treat stack

 * overflow as an unrecoverable SIGSEGV, so we want to error out ourselves

 * before hitting the hardware limit.

 *

 * check_stack_depth() just throws an error summarily.  stack_is_too_deep()

 * can be used by code that wants to handle the error condition itself.

 */

void

check_stack_depth(void)

{

  if (stack_is_too_deep())

  {

    ereport(ERROR,

        (errcode(ERRCODE_STATEMENT_TOO_COMPLEX),

         errmsg("stack depth limit exceeded"),

         errhint("Increase the configuration parameter \"max_stack_depth\" (currently %dkB), "

             "after ensuring the platform's stack depth limit is adequate.",

             max_stack_depth)));

  }

}

bool

stack_is_too_deep(void)

{

  char    stack_top_loc;

  ssize_t   stack_depth;

  /*

   * Compute distance from reference point to my local variables

   */

  stack_depth = (ssize_t) (stack_base_ptr - &stack_top_loc);

  /*

   * Take abs value, since stacks grow up on some machines, down on others

   */

  if (stack_depth < 0)

    stack_depth = -stack_depth;

  /*

   * Trouble?

   *

   * The test on stack_base_ptr prevents us from erroring out if called

   * before that's been set.  Logically it should be done first, but putting

   * it last avoids wasting cycles during normal cases.

   */

  if (stack_depth > max_stack_depth_bytes &&

    stack_base_ptr != NULL)

    return true;

  return false;

}

/* GUC check hook for max_stack_depth */

bool

check_max_stack_depth(int *newval, void **extra, GucSource source)

{

  ssize_t   newval_bytes = *newval * (ssize_t) 1024;

  ssize_t   stack_rlimit = get_stack_depth_rlimit();

  if (stack_rlimit > 0 && newval_bytes > stack_rlimit - STACK_DEPTH_SLOP)

  {

    GUC_check_errdetail("\"max_stack_depth\" must not exceed %zdkB.",

              (stack_rlimit - STACK_DEPTH_SLOP) / 1024);

    GUC_check_errhint("Increase the platform's stack depth limit via \"ulimit -s\" or local equivalent.");

    return false;

  }

  return true;

}

/* GUC assign hook for max_stack_depth */

void

assign_max_stack_depth(int newval, void *extra)

{

  ssize_t   newval_bytes = newval * (ssize_t) 1024;

  max_stack_depth_bytes = newval_bytes;

}

/*

 * Obtain platform stack depth limit (in bytes)

 *

 * Return -1 if unknown

 *

 * Note: we choose to use ssize_t not size_t as the result type because

 * callers compute values that could theoretically go negative,

 * such as "result - STACK_DEPTH_SLOP".

 */

ssize_t

get_stack_depth_rlimit(void)

{

#if defined(HAVE_GETRLIMIT)

  static ssize_t val = 0;

  /* This won't change after process launch, so check just once */

  if (val == 0)

  {

    struct rlimit rlim;

    if (getrlimit(RLIMIT_STACK, &rlim) < 0)

      val = -1;

    else if (rlim.rlim_cur == RLIM_INFINITY)

      val = SSIZE_MAX;

    /* rlim_cur is probably of an unsigned type, so check for overflow */

    else if (rlim.rlim_cur >= SSIZE_MAX)

      val = SSIZE_MAX;

    else

      val = rlim.rlim_cur;

  }

  return val;

#else

  /* On Windows we set the backend stack size in src/backend/Makefile */

  return WIN32_STACK_RLIMIT;

#endif

}